A portable wireless communication device such as a mobile phone and a mobile communication terminal can have a longer operating time by a battery as a result of reducing an average consumption current. The average consumption current is found by multiplying a consumption current for each output power of a radio wave outputted from a wireless communication device via an antenna by an actual frequency in use. A use frequency distribution of DG09, a general index for use frequency, is depicted in FIG. 7A. It is found from FIG. 7A that the use frequency is high at an output power of around 0 dBm, which is a low output power, and current reduction in a neighborhood of 0 dBm is important in order to reduce an average consumption current.
An amplification efficiency of an power amplifier, as depicted in FIG. 7B, reaches maximum at a maximum output power and is not high at a low output power. As a method for raising an amplification efficiency at a low output power to reduce a consumption current and reduce an average consumption current, a power amplifier is used which has a power mode switching function switching amplifier circuit units to be used in correspondence with an output power. The power amplifier having the power mode switching function includes an amplifier circuit unit of a high power mode having a characteristic HPW depicted in FIG. 8A and an amplifier circuit unit of a low power mode having a characteristic LPW of not reacting a maximum output power of the power amplifier but having a high amplification efficiency at a low output power. In other words, the power amplifier having the power mode switching function is, as depicted in FIG. 8B, provided with an amplifier circuit unit (LPM) 302 of the low power mode different from an amplifier circuit unit (HPM) 301 of the high power mode between an input Pin and an output Pout in the power amplifier.
FIG. 9 is a diagram depicting a circuit configuration example of a conventional power amplifier having a power mode switching function. FIG. 9 depicts a circuit configuration example of a last stage (output side) amplifier and an output matching part in a power amplifier of a two stage amplifier configuration (for example, see Patent Document 1). An output side circuit of the power amplifier depicted in FIG. 9 has a low power mode circuit path which includes second stage transistors LPM2P, LPM2N driven at an operation in a low power mode, and a high power mode circuit path which includes second stage transistors HPM2P, HPM2N driven at an operation in a high power mode. These circuit paths are connected in parallel to a primary side of a transformer TRO to a secondary side of which an output load RL is connected.
The low power mode circuit path has an inductor L connected between outputs (drains) of the transistors LPM2P, LPM2N, and capacitors CX connected in series between outputs of the transistors LPM2P, LPM2N and a primary side of a transformer TRO. The high power mode circuit path has capacitors CH connected between outputs (drains) of the transistors HPM2P, HPM2N, respectively. A matching capacitor C1 is connected to the primary side of the transformer TRO in parallel and a matching capacitor C2 is connected to the secondary side of the transformer TRO in parallel.
[Patent Document 1] U.S. Pat. No. 7,728,661
In the conventional power amplifier depicted in FIG. 9, at the low power mode operation, the transistors LPM2P, LPM2N operate and the transistors HPM2P, HPM2N do not operate, but a current flows also to the transistors HPM2P, HPM2N which are not in operation, leading to increase of a power loss. At the high power mode operation, the transistors HPM2P, HPM2N operate and the transistors LPM2P, LPM2N do not operate, but a current flows also to the transistors LPM2P, LPM2N which are not in operation, leading to increase of the power loss. For example, at the low power mode operation, a power indicated by a solid line 401, the power outputted from the transistors LPM2P, LPM2N flows as indicated by a solid line 402, and a part of the power flows to a transistors HPM2P, HPM2N side as indicated by a broken line 403 and lost. As described above, in the conventional power amplifier having the power mode switching function, the transistor not in operation is directly connected to the circuit path in operation, and the power loss increases due to the transistor not in operation.